This invention relates generally to apparatus for reproducing an information signal from a recorded disc rotated thereon, and more particularly, is directed to improvements in a disc player used for reproducing an information signal from a rotating disc having a spiral track on which such a digital signal as an audio PCM (pulse code modulation) signal is recorded with predetermined constant recording density.
In reproducing an information signal from a rotating disc having a spiral track on which a digital information signal containing a frame synchronous signal is recorded with constant recording density, it is required to keep the tangential velocity of the spiral track relative to a pickup device which reads the digital information signal from the spiral track (hereinafter referred to as scanning velocity) constant at predetermined speed whenever the pickup device is located on the spiral track between its innermost circle and its outermost circle provided on the disc. In order to keep the scanning velocity constant, the rotation of the disc is controlled in such a manner that, first the preliminary rotation control state is taken for controlling the rotation speed of the disc to vary in inverse proportion to the radius of a circle of the spiral track on which the pickup is really positioned so that the scanning velocity become almost constant at the predetermined speed, then when a reproduced digital information signal is obtained in the condition of frame synchronism, the phase comparison rotation control state in which the reproduced frame synchronous signal is compared in phase with a reference signal to produce a control signal for the rotation of the disc is taken for controlling the rotating speed of the disc to keep more accurately the scanning velocity constant at the predetermined speed.
As a manner of control during the preliminary rotation control state, it has been proposed that a potentiometer which generates an output varying in response to the position of the pickup device in the direction of the movement of the pickup device be provided for controlling the rotation speed of the disc in response to the output of the potentiometer. In this case, for example, a voltage which varies in inverse proportion with the radius of a circle of the spiral track from which the information signal is read out by the pickup device is produced in response to the output of the potentiometer. The voltage produced is supplied to a voltage controlled ocsillator, the output of which is compared in phase with an output of a frequency generator coupled directly with a motor for rotating the disc, to vary the output of the voltage controlled ocsillator in response to the output of the potentiometer, and the rotating speed of the motor for rotating the disc is controlled by the result of the comparison between the output of the voltage controlled oscillator and the output of the frequency generator so that the output of the frequency generator coincides in frequency and phase with the output of the voltage controlled ocsillator. As a result of this, the scanning velocity is set to be almost constant at the predetermined speed.
However, in this technique for controlling the scanning velocity to be almost constant at the predetermined speed with use of the potentiometer, there are several disadvantages. For example, a circuit performing the dividing operation, which is usually very expensive, is necessary for producing a voltage varying in inverse propotion with the output of the potentiometer. This kind of control system is unstable in the presence of variations in temperature and further the controlable range is restricted to be relatively narrow so that the stable control can not be performed in case the predetermined speed at which the scanning velocity is to be set is changed.
Accordingly, a different control technique described hereinafter has been also proposed for the preliminary rotation control state and used in practice in place of the above described control manner using the potentiometer and having the disadvantages mentioned above.
Generally, in the case of recording a PCM signal wherein the PCM signal is recorded in the form of a base band signal without carrier modulation such as amplitude modulation or frequency modulation, the run length limited code modulation is often adopted. The run length limited code modulation is a modulation system in which two different levels of the recording signal are provided in accordance with each binary data "0" and each binary data "1", respectively, and the minimum interval between two successive level transitions, which is called the minimum run length, Tmin, is made relatively long so that recording efficiency is increased and further the maximum interval between two successive level transitions, which is called the maximum run length, Tmax, is made relatively short so that self-clocking in the reproducing process is facilitated. In addition, since the maximum run length Tmax does not appear continuously in the normal modulated output, a specific pattern of waveform formed with a couple of successive intervals each corresponding to the maximum run length Tmax is added to the modulated output so as to be used as a frame synchronous signal. The maximum run length Tmax is arranged to be, for example, 5.5T (T is a cycle of binary digits of input binary data) and the minimum run length Tmin is arranged to be, for example, 1.5T.
As for previously proposed disc players used for reproducing an information signal from a disc on which a PCM signal is recorded with a waveform obtained by the run length limited code modulation wherein the maximum run length Tmax and the minimum run length Tmin are fixed appropriately, it has been proposed for the preliminary rotation control state that the maximum interval of a constant level between two successive level transitions in the reproduced signal from the disc is detected and compared in length with a reference interval corresponding to the maximum run length Tmax obtained in the condition wherein the disc is rotated with the correct scanning velocity, then in case that the detected maximum interval of a constant level is longer than the reference interval, a motor for rotating the disc is controlled to rotate at higher rotating speed so that the maximum interval of a constant level between two successive level transitions in the reproduced signal from the disc is reduced and, on the other hand, in case that the detected maximum interval of a constant level is shorter than the reference interval, the motor for rotating the disc is controlled to rotate at lower rotating speed so that the maximum interval of a constant level between two successive level transitions in the reproduced signal from the disc is increased, and as a result of this control, the rotating speed of the disc is so controlled that the scanning velocity becomes almost constant at predetermined speed.
Also proposed has been a control technique for the preliminary rotation control state wherein the minimum run length Tmin is utilized for controlling the scanning velocity to be constant. In this control manner, the minimum interval of a constant level between two successive level transitions in the reproduced signal from the disc is detected and compared in length with a reference interval corresponding to the minimum run length Tmin obtained in the condition wherein the disc is rotated with the correct scanning velocity, then in case that the detected minimum interval of a constant level is longer that the reference interval, the motor for rotating the disc is controlled to rotate at higher rotating speed and, on the other hand, in case that the detected minimum interval of a constant level is shorter that the reference interval, the motor for rotating the disc is controlled to rotate at lower rotating speed, so that the rotating speed of the disc is so controlled that the scanning velocity becomes constant at the predetermined speed.
With either control technique mentioned above, when the reproduced signal from the disc is obtained in the condition of frame synchronism due to the control in the preliminary rotation control state, the phase comparison rotation control state wherein a reproduced frame synchronous signal is utilized is taken to control the rotating speed of the disc to keep the scanning velocity constant at the predetermined speed more accurately.
However, in the phase comparison rotation control state, when a pickup device is positioned on a mirror surface, where no signal is recorded, provided at the peripheral portion of the disc for some reason or, in the case of an optical pickup device, defocusing of an optical stylus is caused, the reproduced frame synchronous signal is not obtained and therefore the phase comparison rotation control state can not be continued, so that the preliminary rotation control state is taken again automatically. In such a case, in case that the above mentioned control manner wherein the maximum interval or the minimum interval of a constant level between two successive level transitions in the reproduced signal from the disc is detected to control the scanning velocity to be almost constant at the predetermined speed is adopted for the preliminary rotation control state, since a very long interval of a constant level appears in the reproduced signal, such a control as to intend to shorten the interval of a constant level is performed and consequently the motor for rotating the disc is continuously controlled to increase its rotating speed. This results in the problem that the rotating speed of the motor becomes extremely high and the motor tends to run away.